Method of treating metallic strip with sodium vapor



Aug. 28, 1962 K. G. COBURN METHOD OF TREATING METALLIC STRIP WITH SODIUMVAPOR Filed Aug. 19, 1960 E6. 2. ma

INVENTOR. /(/VN7'// 6 Coaue/v,

ATTORN EVS.

hired EJ351537 NIETHOD OF TREATENG NIETALLIC S WITH SQDlUit/l VAPOR Thisinvention relates to a method of treating metallic strip and the surfaceof the coating bath with sodium vapor in the coating of ferrous metalstrips with aluminum or aluminum alloys by the hot dip procedure.

One of the coating procedures now very generally practiced is one inwhich the metal strip is first passed through an oxidizing furnace toburn oils and other carbonaceous substances from the surface thereof andto form on the strip a microscopically thin oxide film. The strip isthen passed through a reducing furnace in which the thin oxide film isreduced so as to leave the surfaces of the strip absolutely clean, inwhich state they are particularly receptive to the molten coating metal.The strip is then led through a hood under the protection of a neutralor reducing atmosphere and without a flux treatment into the coatingbath. The protective hood dips into the coating bath so that the stripis at no time exposed to the atmosphere. The above outlined procedure istaught in the Sendzimir Patent No. 2,110,893.

When the coating metal is aluminum or an aluminum alloy, difficultieshave been encountered in that deposits form on the surface of thealuminum bath and are picked up by the strip and these depositsadversely afiectthe adhesion of the molten coating metal. This problemwas recognized and an antidote therefor was disclosed and claimed in theOganowski Patent No. 2,437,919. In accordance with the teachings ofOganowski, a bell was provided at the end of the hood which dipped intothe coating bath and the bell was provided with an annular troughsurrounding the strip and into this trough metallic sodium was fed.Since the trough was subjected to the heat of the bath, the metallicsodium was therefore melted and vaporized.

Oganowski suggested that various substances such as nitrides were formedby the interaction of the bath with the nitrogen of the dissociatedammonia protective atmosphere in the hood and might also include oxidesand hydrides formed by the interaction of various substances presentwith water vapor. He suggested that these nitrides, oxides and hydridesformed a scum on the bath which was dragged down into the bath on thestrip which resulted in surface defects.

Oganowski suggested that it was possible to form on the surface of thebath a powdery coating which would be non-adherent to the strip andwhich would to a very large degree prevent interaction of the bath andatmospheric constituents and in any event render the strip non-adhesivewith respect to such nitrides, oxides and the like which might beformed. Oganowski taught that one could form upon a surface of analuminum bath a layer which consisted mainly of powdery sodiumaluminate, by introducing sodium vapors adjacent the surface of themolten aluminum within the hood.

There are certain disadvantages involved in generating the sodium vaporin the hood by the heat of the bath in that the melting and vaporizingtemperature is roughly that of the coating bath which is maintainedrelatively constant. Therefore, sodium vapor is only generated at aconstant rate. If it is attempted to add metallic sodium more rapidly,there is a tendency for liquid sodium to splash directly onto the stripand this also produces defects in the coating. Furthermore, when it isremembered 3,@5l,58? Patented Aug. 28, 1962 that the hood containshydrogen or dissociated ammonia, it will be understood that sodiumreacts rapidly with hydrogen to form sodium hydride and therefore thesodium in the trough is rapidly converted to an inactive product so thatthere is a substantial waste of sodium.

Oganowski suggested that sodium vapors might be generated elsewhere andintroduced into the hood, but indicated that this was diflicult if notimpossible because of the dilution of the sodium vapors by theprotective atmosphere and because of the likelihood of turbulence withinthe hood.

With the foregoing considerations in mind, it is an object of thepresent invention to provide a method of treating a strip, which is tobe coated with aluminum or aluminum alloy, with sodium vapor wherein thesodium vapor is generated outside the hood and is conveyed to the hoodby means of a carrier gas. It is another object of the invention toprovide for a control of the sodium vapor addition. and to provide forgreater uniformity in the rate of sodium vapor additions. It is also anobject of the invention to eliminate the danger of sodium splash byremoving solid and liquid sodium from the immediate vicinity of thestrip being coated.

These and other objects of the invention which will be apparent to oneskilled in the art are accomplished by that method of which an exemplaryembodiment will now be. described.

Reference is made to the drawings forming a part hereof and which willillustrate more or less diagrammatically an apparatus by means of whichthe method may be carried out.

In the drawings,

FIGURE 1 is a diagrammatic view of a vaporizer and coating bath showingthe relationship between the two.

FIGURE 2 is a cross-sectional view through an exemplary vaporizer; and

FIGURE 3 is a fragmentary cross-sectional view on the line 3-3 of FIGURE1.

Briefly, in the practice of the invention there is provided a coatingpot which may be conventional with a conventional hood according to theSendzimir patent, dipping into the molten coating metal and providing aprotective passage for the strip from the reducing furnace into thecoating bath. A vaporizer is provided with means for heating thevaporizing chamber and means are provided for adding metallic sodium tothe vaporizer. A carrier gas is caused to flow into the vaporizer andthrough the vaporizer, whereby to entrain vaporized sodium and a conduit.is provided to carry the carrier gas and its entrained sodium vaporfrom the vaporizer to the hood.

Referring in greater detail to the drawings, a coating pot is generallyindicated at 10 and contains a body of molten coating metal 11. Withinthe pot there is mounted the usual pot roll 12 aroundwhich the strip 13passes. The hood 14 extends from the exit of they reducing furnace downinto the bath 11 and means are provided (not shown) for feeding into thehood a protective atmosphere which will usually be dissociated ammonia.It will be understood that the strip, in passing through the hood 14,has been pretreated so that it is absolutely clean and it then passesinto the coating bath, around the .pot roll 12 and issues from the bathand is permitted to cool.

In FIGURE 1 a vaporizer is indicated generally at 15. A carrier gas isfed into the vaporizer through a conduit 16 from a suitable source (notshown) and the carrier gas with its entrained sodium Vapor passesthrough a conduit 17 from the vaporizer 15 to the hood 14. Metallicsodium is introduced into the vaporizer through the pipe 18 which isprovided with a pair of single action, wide opening valves 19a and 19b.By means of these requirements.

J resides -in the, particular carrier gas which is used.

valves it is possible. to introduce sodium into the gvaporizer withoutsucking air into it. Referring now to FIGURE 2, the vaporizer comprisessodium '22. .,Under the-influence of the heat provided by itheelectricheating elements 21,-.the liquidsodium is :vaporized and'the balance ofthe vessel .29 is filled with sodium-vapor as indicated at 243. Abafile'24is provided the sodium ,vapor passes into the hood and build upof frozen metal or high melting point reaction products within the hoodhas been very greatly reduced. Elimination of the sodium trough of theQganowski patent permits better heat transfer from the aluminum bathoutside thehood, through the metal hood, so as to. pre- 1 vent freezingof the coating metal within the hood. The

1o cause the carrier gas entering through the conduit 16- to'flowdownwardly around the baflle, whereby .to pick up and entrain-vaporizedsodium. The carrier gas with the entrained sodium vaponthen passesthrough the con- Iduit .17 which-may be heated asby means of heating:

elements .17, -,and/or.insulated asat 17d, to prevent condensationof thevapor before it reaches the hood. The vessel.20;is enclosed within aninsulating container so as to minimizeheatloss to economize on heat Asseen in FIGURE3 where a' strip is beingicoated, V

the conduit ,17 :is provided ,with two branches 17a and 17b preferablyentering from opposite sides .of the 'hood and "disposed adjacent thesurfaceof the coating bath at a small angleofjincidence withrespecttothe strip so as .to cause the sodium vapor topass across thesurface ofthebath andthe strip with a ofturbulence to achievethe resultsoutlinedabove.

invention It might vbe :thought thatbecause the hood contains,dissociated ammonia it would be desirable to use dissociated One veryimportant aspect of the present lammonia as the carrier for thesodium'yapor. This,

however, is not so because. the use of dissociated ammonia results intheformation of sodium -hydrides which wouldtendto plug up the. conduit17.

According to the present invention, the carrier gas used is nitrogenwhich .is completely inert with respect to 7 sodium, and which has theadvantage ofbeing heavier. than hydrogen .so that the hydrogen-which ispresent in V the dissociated ammonia, atmosphere in the hood isdisplaced upward by the heavier-nitrogen}carrier gas. "Fhus,

:while-asmall amount of hydrides maybe formed, this hydride formationwill be'negligible.

- It willnow be nnderstoodthatthe uniformity of sodium .vapor. additionmay. be .very accurately controlled by a 1 controlv of the vaporizingheat-appliedthrough theheating elementsll, by therateof additionofmetallic sodium and-by the rate of flow of the carrier gas.

.Without limitation but by waytof. example, the above-disclosedprocedure' has been practiced using a tempera, ,turebetween11000 F. and 1100 F. and a flow of nitroj gen offlSOcubic-feetper hour. It be understood I that the temperature must be .suflieient toheat the carrier gas as well as toiprovidefonthe melting and vaporiza- Qtionof the sodium. When the carrier gas is .used in large volume,preheating thereof-ahead of the sodium.-\' aporizing chamber isadvisable, as by'means of heating elements ,16a. Y e 7 It alsobe clearthatby removing the. source of the sodium vapor from .thehood andthestrip passing 'therethrough, the problem of .sodium splash has beenentirely eliminated. The use of nitrogenas a carrier gas has eliminatedthe plugging up of the linesithrough which sive that it is the carrierof choice.

It will be understood that various modifications may be made withoutdeparting from the spirit of the invention and therefore no limitationnot specifically set forth in the claims is intended.

7 What is claimed is: V V V '1. In a process of coating a ferrous metalarticle with molten coating metal of a class consisting of aluminum andaluminum alloys, in whichthe metal article is passed through a hood intoa bath of said moltencoating metal, and in'which' a protectiveatmosphere which issubstantially non-oxidizing to ferrous metals ismaintained in said hood; the steps of generating sodium vapor outsidesaid hood, conveying said sodium vapor, by means of a carrier stream ofnitrogen gas, intosaid hood adjacent the surface of said molten coatingmetal, whereby to subject the surface of the molten coating metal andthe surfaces of the metal article being coate d,'to the action of sodiumvapor. V 7

2.. .A process according to claim 1, wherein the protective atmospherein said hood is dissociated ammonia.

3. A process according to claim 1, whereinthe article being coated is astrip of ferrous metal, and said sodium vapor, inits nitrogen gascarrier, is directed at both sides of said strip, adjacent the surfaceof the molten coating metal, at a small angle of incidence.

4. .In a process of coating a ferrous metal articlewith molten coatingmetal of a class consistingof aluminum and aluminumalloys, in which themetal :article is passed through'a hood into a bath of said moltencoating metal, and in which a protective atmosphere whichissubstantiallyhon-oxidizing to ferrous metals is-maintained. in saidhood; the'step of continuously subjecting the surfaces of said articleand of the moltencoating metal within said hood,'to the action of sodiumvapor entrained ina stream of nitrogen gas. e j r 5; YA processaccording toclaim 4, wherein said article being coated is a strip offerrous metal, and said sodium vapor, in its nitrogen gas carrier,is'dire cted at both sides of said strip adjacent thesurface ofthe'molten coating metal, .ata small angle of incidence.

tive atmosphere in said hood is dissociated ammonia.

V 1 References Cited in the file of this patent 7 UNITED sT TEsPATEnT s2,437,919 Qrganowski Mar. 16,1948 2,503,571 Wehe Apr. 11, 1950 2,785,651Pawlyk" Mar. 19, 1957 2,914,419

Organowski Nov. 24, 1959

1. IN A PROCESS OF COATING A FERROUS METAL ARTICLE WITH MOLTEN COATINGMETAL OF A CLASS CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS, IN WHICHTHE METAL ARTICLE IS PASSED THROUGH A HOOD INTO A BATH OF SAID MOLTENCOATING METAL, AND IN WHICH A PROTECTIVE ATMOSPHERE WHICH ISSUBSTANTIALLY NON-OXIDIZING TO FERROUS IS MAINTAINED IN SAID HOOD; THESTEPS OF GENERATING SODIUM VAPOR OUTSIDE SAID HOOD, CONVEYING SAIDSODIUM VAPOR, BY MEANS OF A CARRIER STREAM OF NITROGEN GAS, INTO SAIDHOOD ADJACENT THE